added 'NA' and 'CL' to set {'+', '-'} identifying ions

[kmboehm]

Addressing issue #1026

[andrrizzi]

Thanks for the PR! A couple of suggestions:

1. It looks like you've configured your IDE to use tabs instead of spaces. We shouldn't mix the two types of indentation in Python. Can you switch to 4-space tabs the settings?
2. Using the in operator here risks to collect things we don't want. What about having maintaining a set of residue names we want to consider ions (beside the ones containing + and -) and have something like

ION_RESIDUE_NAMES = {'NA', 'CL'}
return [i for i in self._solvent_atoms
    if '-' in self._topology.atom(i).residue.name or
    '+' in self._topology.atom(i).residue.name or
    self._topology.atom(i).residue.name in ION_RESIDUE_NAMES]

This way we check for equality instead of looking in the substrings.

[andrrizzi]

Looks great!

[jchodera]

Looks good!

I was trying to think of whether we could easily expand our test coverage for these cases too, but I'm not sure that would be easy right now, so I'd just go ahead and merge.

[kmboehm]

Thanks, guys!